Maintains a HashMap of SummaryStatistics instances with a composite
key of the form (process,type). "Process" typically identifies the client
thread and "type" identifies the metric - e.g., "latency", "numActive."

getStatisticsByProcess(String process)
Returns the full list of SummaryStatistics corresponding to
the given process - i.e, the list of statistics of
of different types maintained for the given process.

getStatisticsByType

Returns the full list of SummaryStatistics corresponding to
the given type - i.e, the list of statistics of the
given type across processes. For example,
getStatisticsByType("latency") will return a list of latency
summaries, one for each process, assuming "latency" is the name of
an accumulated metric.

getMeanSummary

Returns a SummaryStatistics instance describing the mean of the given
metric across processes - i.e., the "mean of the means", the
"min of the means" etc. More precisely, the returned SummaryStatistics
describes the distribution of the individual process means for the given
metric.

The same results could be obtained by iterating over the result of
{getStatisticsByType(String) for the given type,
extracting the mean and adding its value to a SummaryStatistics
instance.

Parameters:

type - the metric to get summary mean statistics for

Returns:

a SummaryStatistics instance describing the process means for
the given metric